With regard to the conventional prior art, the U.S. Pat. No. 4,165,717 is referred to, which presents a circulating mass process with a particle separator located outside the furnace and with the return of solid material through a return duct to the lower part of the furnace. The return duct is not, however, equipped with a gas lock.
With regard to the prior art, further reference can be made to the publication F.A. Zenz, "Fluidization and Fluid-Particle Systems", Pemm-Corp Publications, Vol. II, Draft, 1989, pp. 333-334. This publication deals with a so-called bubbling fluidized bed reactor, in which the particle separator is placed inside the reactor chamber and the return duct for solid material is vertical and concentric with the joint center axis of the reactor chamber and the particle separator. The lower part of the return duct comprises a gas lock arrangement which is particularly described on page 334 of this publication. The gas lock arrangement comprises mechanical mobile parts, and consequently, it is extremely difficult to be constructed under the exact conditions of the reactor chamber. This fact naturally results also in that a gas lock arrangement of this type is very unreliable in function.
Moreover, with regard to the prior art, reference is made to U.S. Pat. No. 5,070,822 and European Patent Application Publication No. 440,552. Some other improvements in circulating mass process are presented in these prior publications that can be applied also in a combustion unit according to the present invention if necessary.
In brief, it can be stated on basis of the knowledge of the prior art that gas lock structures in the prior art have been unsatisfactory with regard to their structure as well as their function